FRIDAY, Sept. 18, 2015 (HealthDay News) -- Smokers have a much greater risk for type 2 diabetes than those who never smoked, and the same is true for those routinely exposed to secondhand smoke, a new study suggests.

But the Harvard researchers said this increased risk gradually drops over time once smokers kick the habit.

"Cigarette smoking should be considered as a key modifiable risk factor for diabetes," study co-author Frank Hu, a professor of nutrition and epidemiology at the Harvard T.H. Chan School of Public Health in Boston, said in a university news release.

"Public health efforts to reduce smoking will have a substantial impact on the global burden of type 2 diabetes," Hu added.

Countless studies have linked smoking to serious health issues, including cancer, lung disease and heart disease. In conducting the study, the researchers investigated the link between smoking and diabetes. The study was only designed to find an association, and not a cause-and-effect relationship.

The new analysis included 88 previous studies involving nearly 6 million people. The studies specifically examined the effects of smoking on the risk for type 2 diabetes.

Compared with never smoking, current smoking increased the risk for the disease by 37 percent, according to the report published Sept. 18 in The Lancet Diabetes & Endocrinology.

Former smokers were also at 14 percent greater risk for type 2 diabetes than those who never smoked, and people exposed to secondhand smoke on a regular basis had a 22 percent higher risk for the blood sugar disease, the findings showed.

But the investigators also found that smokers who quit reduced their risk for type 2 diabetes. The increased risk was up to 57 percent (depending on how much one smoked) before quitting, 54 percent within five years of quitting and 18 percent after five years of quitting. Once a decade had passed, former smokers' increased risk for type 2 diabetes dropped to 11 percent, the study found.

The study authors estimated that nearly 12 percent of all cases of type 2 diabetes in men and over 2 percent of all cases in women (almost 28 million cases worldwide) may be linked to active smoking.

"Despite the global efforts to combat the tobacco epidemic, cigarette use remains the leading cause of mortality and morbidity worldwide," study first author An Pan, a professor of epidemiology at the School of Public Health at Tongji Medical College, Huazhong University of Science and Technology, in China, said in a university news release.